Answer:
5.7
Step-by-step explanation:
3x+4 = 21
3x = 17
x = 5.666667 = 5.7
Depends on MPH or Miles Per Hour.
Going 10 mph = 30 minutes
Going 50 mph = 10 minutes
Etc.
I dont know if this is right but the answer to 2(12+8x)-3(4x-9) might be 4x+51?
I think its 200%
Best Of Luck!!!!
:D